The oriented electrical steel sheet has a texture in which the crystal grains have (110)[001] orientations. This product has extremely superior magnetic properties in the rolling direction, and by utilizing these properties, the product is used in transformers, electric motors, electric generators and other electric and electronic apparatuses as steel cores. This (110)[001] texture is obtained through a secondary recrystallization, and the secondary recrystallization grains are formed in such a manner that particularly oriented nuclei, i.e., nuclei having a (110)[001] orientation among the fine primary recrystallization grains are abnormally grown throughout the entire test piece. If such a secondary recrystallization having the particular orientation are to be promoted, the growth of the primary recrystallization grains has to be inhibited, and, for this purpose, precipitates of MnS, AlN and BN are used.
The oriented electrical steel sheet can be classified into two kinds based on the magnetic properties. The first is the conventional oriented electrical steel sheet which was almost perfected in 1960's, since Armco Company of the United States began industrial production from the early 1940's. This oriented electrical steel sheet shows a deviation of about 7.degree. of the orientation [001] relative to the rolling direction, while its magnetic flux density is about 1.80-1.86 Tesla at B10. The second kind is that which was developed by NSC company of Japan in 1970's, and this oriented electrical steel sheet shows a far more superior magnetic properties than the first kind. That is, the magnetic flux density is over 1.89 Tesla at B10, and the orientation [001] shows a deviation of only 3.degree..
Generally, the oriented electrical steel sheet is manufactured in the following manner. Generally, the steel slab contains 2-4% of silicon, and also contains grain growth inhibitors such as MnS, MnSe, and AlN. Then the steel slab undergoes complicated process steps: reheating and hot rolling - preliminary annealing first cold rolling - intermediate annealing - second cold rolling - decarburization annealing - coating of an annealing separator -final finish annealing - tension coating treatment. This is a first method, and a second method is carried out in the following manner. That is, first a preliminary annealing is carried out, then one-stage of strong rolling is carried out to the final thickness, and then, a decarburization annealing is carried out.
In the complicated manufacturing process for the oriented electrical steel sheet, the most difficult process step is the reheating step in which the steel slab is subjected to a high temperature heat treatment. In carrying out this steel slab reheating step, the precipitates of MnS or AlN are made to be finely precipitated after solid-solution spreading. In order to achieve this, it is unavoidable that the slab has to be heated to as high as 1400.degree. C., and has to be maintained at this temperature for 5 hours. Under this condition, the slab surface is reacted with the air, and consequently, there is formed an oxide called faylite (2FeO.multidot.SiO.sub.2) which is a composite material composed of Si oxides and Fe oxides. This oxide has a melting point as low as 1300.degree. C., and therefore, its melt flows down along the surface of the slab. This oxide melt is designed to flow down to the outside, but a part of it is accumulated within the refractory material of the upper portion of the furnace. Therefore, upon completion of the slab reheating step, the interior of the furnace has to be repaired. Therefore, in a steel plant in which a continuous working is required, the work efficiency is lowered, the productivity is lowered, and the manufacturing cost is increased. Therefore, if it is possible to reheat the slab only to 1250.degree.-1280.degree. C. which is the heating condition for the ordinary steel, then the interference to the general steels can be eliminated, as well as obtaining other advantages. This will be a great advantage.
The lowering of the slab reheating temperature is intensively researched by many steel manufacturing companies, and many methods have been proposed. Most of the proposals aim at ensuring that the required magnetic properties are obtained by reheating the steel slab up to below the highest slab non-melting temperature (1300.degree.-1320.degree. C.) in principle.
Most of the proposals are focused on adjusting the basic composition, and on selecting a proper grain growth inhibitor which contributes to the formation of the secondary recrystallization of the (110)[001] orientation. In addition to the adjustment of the basic composition, the precipitate managing techniques are also proposed.
These proposals are found in Japanese Patent Laid-open No. Hei-1-230721, Hei-1-283324, Hei-4-235222, hei-4-173923, hei-3-294427, Sho-59-56522, and sho-59-190325.
All of them provides an oriented electrical steel sheet showing a flux density of over 1.89 Tesla at B10. The target slab reheating temperature is below 1200.degree. C., and usually the slab is heated only up to 1150.degree. C. Further, in order to ensure a high magnetic flux density, the N ingredient which is a grain growth inhibitor is supplemented during the process. For this, there is added the nitrogenizing treatment to infuse nitrogen into the slab after carrying out the decarburization annealing. Therefore, in these techniques, the slab reheating is carried out at a temperature lower than 1250.degree.-1280.degree. C. which is the heating temperature for the ordinary steel. Therefore, there occur interferences, and an additional facility for the nitrogenizing treatment has to be built, thereby increasing the manufacturing cost. Further, in spite of the low reheating temperature, one-stage of a strong rolling is adopted for ensuring a strong grain growth inhibiting force, and therefore, the process control becomes difficult, with the result that severe differences between different lots are generated, thereby lowering the actual yield. Therefore, this low temperature heating method can be applied to only the expensive product.
Meanwhile, ALSCO Company of the United States reported that an electrical steel sheet can be manufactured by properly controlling the Mn/S ratio based on the low temperature reheating condition of Japanese Patent Laid-open No. Sho-57-158322. However, in this method, the control of the grain growth inhibition strength is difficult, and therefore, stable magnetic properties cannot be obtained.